(A) Field of the Invention
The present invention relates generally to a keyboard for an electronic device which uses a row-column type structure to detect a key input and in particular to such a keyboard which detects a key input without continuous scanning of the rows or columns.
(B) Background Art
Traditionally, keyboards for various electronic devices with many key inputs use a row-column type structure consisting of a matrix of keys wherein each key is defined as the intersection of a row and column wire. A switch is used at each such intersection to short the rows and columns. The column wires are set to either high or low one at a time with the columns always connected to drivers, and the rows are then used as inputs to see if any of the intersections are shorted in that column.
This type of keyboard has an advantage in minimizing hardware when there are a large number of keys to be used in the design. However, problems have occurred in the prior art in that the continuous nature of the scanning and the long wires required in the row-column structure can lead to RFI (radio frequency interference) transmission from the keyboard. Any such continuous pattern can be detected by an eavesdropper and/or may fail to pass governmental agency RFI tests, thus requiring special shielding over the keyboard or other measures to reduce RFI transmission. One solution to this RFI transmission from the keyboard is to enclose the keyboard in metal. Plastic enclosures are, however, much less expensive and lighter weight to use so that what is desired is the reduction of RFI transmission by internal means so that a metal enclosure is not required.
One prior art attempt to solve the problem maintains all drivers (columns) energized and waits for any input. When there is an input as a key is pressed one scan cycle is completed. A problem occurs in this situation in that with one scan cycle there is still radiated energy back into the electronic device. For example, if the electronic device is a test instrument and the device has not been protected against radio frequency energy the RFI from the one scan cycle can distort a measurement and yield an incorrect result. Further, eavesdropping is still possible on one scan cycle because of the RFI transmitted.
Another prior art approach called "clock dithering" requires that the frequency or time between successive energizings of the drivers (columns) be modulated. In this way, the amplitude of the energy is not centered at one frequency but instead spread over a broader frequency range at a lower amplitude. The problem with this approach is that it is difficult to accomplish and it only reduces but does not eliminate the problem.
U.S. Pat. No. 3,902,054 to Cochran et al., discloses a calculator system having a keyboard with an array of keys in rows and columns. The columns are continuously strobed in sequence by cycle times of the calculator system.
U.S. Pat. No. 4,502,038 to Lowenthol et al., discloses a method and apparatus for continuously scanning a keyboard which allows the simultaneous depression of a multiple number of keys on a keyboard.